This invention relates to a frequency modulator and a transmitter and transceiver incorporating a frequency modulator for a digital radio communication device.
Many digital radio communication systems, such as cellular, cordless and data transmission systems, use FSK, GFSK or GMSK modulation techniques. These types of modulation techniques are in fact simply frequency modulation (FM) with the radio frequency (RF) signal envelope constant.
Since there is no amplitude modulation (AM) involved in these types of modulation, the voltage controlled oscillator (VCO) frequency of the transmitters can be directly modulated by the baseband signal, as is typical in regular analog FM transmitters, such as in analog cellular systems. Significant cost reductions can be obtained by directly modulating the VCO frequency so that such arrangements are particularly desirable in digital applications where low cost is a strategic factor. For example, the overall cost of a digital solution such as DECT must be very low to be competitive with well known analog systems such as CT0.
For analog systems, the voice spectrum, 300 to 3000 Hz or more, is compatible with the lock-up time of the PLL synthesizer when the modulation is applied to the VCO. The lock-up time is generally never less than 5 or 6 ms. However, when applying this low cost technique in digital systems, the baseband signal, which modulates the VCO, is the filtered No-Return-to-Zero (NRZ) data stream coming from the logic section of the transmitter device. This baseband signal has a very low frequency content in its spectrum (from a few Hz). Since the required lock-up time for the PLL synthesizer is around few microseconds, it is not compatible with the spectrum of the baseband signal. The low frequencies of the modulation are considered like VCO drifts and so are corrected by the PLL loop. This incompatibility problem means that either the low frequency content of the baseband signal is lost or the speed of the PLL loop, which determines the speed for channel selection, is compromised.
Thus, before this low cost technique can be advantageously applied in digital systems, the problem of ensuring compatibility with the PLL synthesizer lock-up time needs to be resolved and further requirements have to be considered.
For example, in order to avoid any inter symbol interferences which could corrupt the eyes pattern and degrade the Bit Error Rate of the transmission in the digital system, the group delay on the modulation path has to be kept constant in all the spectrum of the baseband signal.
Furthermore, in order to meet the applicable radio specifications, the spectral purity of the RF signal source to be transmitted should be maintained as near the carrier signal as possible for phase noise and modulation accuracy, and as far from the carrier signal as possible for harmonics, noise floor, discrete spurious signals. Should this last requirement be sufficiently met, filters and duplexers would no longer be needed which would provide a drastic reduction in cost of the transmitter. Being able to reduce the filters and duplexers is significant in keeping the overall cost of the digital system down which improves the competitiveness of the digital solution compared to the analog solution.
Several modulation techniques which modulate the VCO in a digital system are known.
For example an I/Q modulator has been used in a direct or heterodyne architecture. This type of modulator is driven by two RF signals in quadrature coming from an external oscillator which is tuned on the transmitted frequency (direct architecture) or on an intermediate frequency (heterodyne architecture) and by two complex baseband signals, calculated from the data stream (I and Q signals). With such architectures, the RF oscillator(s) or synthesizer(s) are independent of the modulation process so that the lock-up time can be chosen independently from the baseband signal spectrum. However, such architectures are complex and require an expensive xe2x80x98I/Qxe2x80x99 interface in the baseband which can be justified for very accurate GMSK systems, like GSM, but not in standards like DECT or CT2, where the modulation index can vary by large amounts and where cost is strategic. A further disadvantage with these techniques it that they are noisy and so do not allow for the filters and duplexers to be removed.
A different but well known technique uses a heterodyne arrangement with a fixed modulated local oscillator. A channel synthesizer which meets the lock-up time requirements is mixed with a crystal oscillator or a very slow synthesizer able to handle the modulation. This type of arrangement is very efficient and robust and allows zero blind slots because the channel synthesizer is fast enough. In other words, with this type of arrangement every slot can be used. However, it always generates many mixing products out of the bandwidth of the signal to be transmitted, which have to be strongly filtered, increasing the cost of the transmit path.
Another technique called the xe2x80x98open loopxe2x80x99 technique has been used with cordless standards such as DECT or CT2. As mentioned above, a problem with applying the modulation to the VCO is that the low frequency part of the FM modulation will be corrected by the loop of the synthesizer as it does for frequency drift of the VCO. In order to avoid this, the open loop technique opens the loop during a transmission slot so that the VCO is working in a free-run mode without any feedback and can be modulated without any correction from the loop. After the transmission slot, the loop is closed again and the VCO resynchronized. Resynchronization is however difficult to achieve. In fact, it is so difficult to achieve, that the VCO must typically work at half the final frequency and must be followed by a frequency doubler to be properly buffered against transients and frequency drifts. This requires many filters which increases the overall cost of the transmitter device. A further disadvantage of the open loop technique is that it does not allow for zero blind slots because of the frequency drift due to the free-run mode.
In addition, the phase detector of the synthesizer for the open loop technique is very hard to design because of the low leakage current required.
In fact, the open loop technique cannot be used in xe2x80x98high-endxe2x80x99 standards, such as GSM or MOBITEX, where parasitic drifts due to the free-run mode periods are not allowed by the respective specifications. The open loop technique can be implemented for standards such as DECT and CT2, but due to its difficult resynchronisation, only half of the available slots can be used. This is a problem for public base stations or for range improvement.
There is therefore a need to provide an improved low cost frequency modulator for a digital radio communication device which overcomes the above described problems.
In accordance with the present invention there is provided a frequency modulator for modulating a carrier signal according to a modulation data signal to provide a modulated output signal, the frequency modulator comprising:
a reference signal generator coupled to receive the modulation data signal for generating a reference signal modulated according to the modulation data signal; and
a main synthesizer coupled to receive the modulated reference signal and the modulation data signal for providing the modulated output signal at an output, and wherein the reference signal generator comprises an auxiliary synthesizer having a lock-up time which is substantially greater than that of the main synthesizer, the auxiliary synthesizer comprising:
an auxiliary phase detector having a first input for receiving a fixed reference signal having a fixed frequency, a second input and an output for providing an error signal representing the difference in phase between the fixed reference signal and a signal at the second input; and
an auxiliary VCO coupled to receive the error signal and the modulation data signal for generating the modulated reference signal at an output, the output of the auxiliary VCO being coupled to the main synthesizer and to the second input of the auxiliary phase detector.
Thus, by modulating the reference signal supplied to the main synthesizer with the modulation data signal and by applying the modulation data signal to the main synthesizer, the present invention ensures that the modulation process to provide the modulated output signal is independent of the speed of the main synthesizer. This means that the low frequency components of the modulation data signal are not lost nor is the speed of the main synthesizer compromised.
Preferably, the modulated reference signal has a first modulation gain and the modulated output signal has a modulation gain which is substantially proportional to the first modulation gain. This results in the modulated output signal RFout having a response which is flat in amplitude and group delay, and independent of modulating frequency.
In a preferred arrangement, the main synthesizer for generating the modulated output signal and for controlling the frequency of the carrier signal, comprises:
a phase detector having a first input for receiving the modulated reference signal, a second input and an output for providing an error signal representing the difference in phase between the modulated reference signal and a signal at the second input;
a VCO coupled to receive the error signal and the modulation data signal for generating the modulated output signal at an output of the VCO; and
a variable divider coupled between the output of the VCO and the second input of the phase detector for dividing the modulated output signal by a selectable value so as to vary the frequency of the carrier signal. With such an arrangement, the first modulation gain is preferably arranged to be substantially equal to the gain of the modulated output signal divided by the selectable value of the variable divider.